The invention relates to spatially resolved spectroscopic recordings using nuclear magnetic resonance (NMR) methods. Such methods are known as chemical shift imaging (CSI) wherein nuclear spins are excited by means of radio frequency (RF) pulses and a signal thereof is recorded after a certain time during which spatial encoding of the signal by means of a gradient pulse in at least one spatial direction is imposed, wherein said gradient pulse is varied successively from one recording step to the next to encode spatial position of multiple measuring volumes. A method of this type is known e.g. from the publication by T. R. Brown et al. “NMR chemical shift imaging in three dimensions”, Proc. Natl. Acad. Sci. USA, Vol. 79, 3523–3526 (1982).
In aforementioned method, excitation steps are repeated at a repetition time interval TR, wherein TR is typically chosen to be in the order of the longitudinal relaxation time T1, which is much longer than the time available for signal sampling which is given by the transversal relaxation time T2. Accordingly, these measuring schemes are time inefficient.
Steady state free precession (SSFP) techniques are known to be very efficient data sampling schemes with typical time intervals TR less than 1/100 of the relaxation time T1. These schemes were introduced to NMR long ago. Carr was the first to demonstrate the concept (H. T. Carr “Steady-state free precession in nuclear magnetic resonance”, Physical Review, Vol. 112, 1693–1701 (1958)), but it only gained practical relevance with the availability of fast switching gradients. The frequency response in SSFP is periodic in frequency-space with 1/TR and shows wide regions of high signal, so-called pass-bands, and narrow regions of low signal which are referred to as dark-bands or stop-bands (FIG. 1). Accordingly, the TR is typically very short in the order of 2–10 ms to allow recordings over a frequency range of 100–500 Hz. The optimal excitation angle depends on T1 and T2 values but is typically in the order of 40 to 110 degrees. Variants of the SSFP method for spectroscopic recordings are described in U.S. Pat. No. 6,677,750 and U.S. Pat. No. 6,307,368. Other steady state spectroscopic imaging techniques have been proposed for water-fat separation, see: O. Speck, et al. “Fast 31P chemical shift imaging using SSFP methods”, Magn. Reson. Med., Vol. 48, 633–639 (2002), and Dreher, et al. “Fast proton spectroscopic imaging using steady-state free precession methods”, Magn. Reson. Med., Vol. 50, 453–460 (2003).
One practical drawback when applying short TR sequences relates to power deposition in the object, which is of particular relevance in the in-vivo application. The increase in power deposition with increasing field strength can be approximated to scale with the field strength squared.